Magmatic immiscibility and the origin of magnetite-(apatite) iron deposits

The origin of magnetite-(apatite) iron deposits (MtAp) is one of the most contentious issues in ore geology with competing models that range from hydrothermal to magmatic processes. Here we report melt inclusions trapped in plagioclase phenocrysts in andesite hosting the MtAp mineralization at El Laco, Chile. The results of our study reveal that individual melt inclusions preserve evidence of complex processes involved in melt immiscibility, including separation of Si- and Fe-rich melts, the latter hosting Cu sulfide-rich, phosphate-rich, and residual C-O-HFSE-rich melts, with their melting temperature at 1145 °C. This association is consistent with the assemblages observed in the ore, and provides a link between silicate and Fe-P-rich melts that subsequently produced the magnetite-rich magmas that extruded on the flanks of the volcano. These results strongly suggest that the El Laco mineralization was derived from crystallization of Fe-P-rich melts, thus providing insight into the formation of similar deposits elsewhere.


Comment on FEG-EPMA totals
There are no universally accepted values for "what total indicates a good electron probe microanalysis (EPMA) analysis" although typically values between 99% and 101% are considered "acceptable", or "good"."This is in spite of the fact that any total over 100% is physically impossible.The calculation of concentrations of particular analytes is subject to a variety factors (e.g., choice of standards, data reduction method, choice of mass absorption coefficients) and gets even more complex when all elements are not measured (for silicates, typically O, which makes up close to 50% of the total) but are added by some assumed (typically stoichiometric) method.While it is possible to calculate various analytical errors associated with an analysis 1,2 this is rarely rigorously done.
Nonetheless, common practice in the EPMA community is to strive for analytical totals in the 99-101% range, recognizing that there are problems when working with non-ideal samples, which include rough surfaces on samples, areas at the edges of grains where they may be slight "rounding", or grains that are smaller than the interaction volume of the electron beam in the sample.
A field emission gun (FEG)-EPMA (as used here) is capable of creating a beam that is nanometers in width at the surface of the sample.However, the beam spreads when it enters the sample limiting the size of the volume that can be analyzed.Reducing the accelerating voltage (as we did here) reduces the size of the interaction volume, but also limits X-ray production.Attempts to increase X-ray production by increasing beam current produced element migration; indeed element migration was a problem in silicates even with traditionally normal beam currents, most likely because of the greatly reduced volume of interaction.Similar problems were found when increasing counting times; elements migrated in and out of the sample in rather unpredictable ways.The end result was that we obtained fewer X-ray counts than we would have liked, and the resulting analytical spread in the data is greater than for more traditional EPMA W or LaB6 EPMA analyses.The choice of cut off values of 96-104% is to some extent arbitrary, but is based on the observation that within this range the measured atomic fractions within each sample type is relatively stable.Abbreviations: BF image -bright field image, HRTEM -high resolution TEM, HAADFhighangle annular dark field image, FFT -Fast Fourier Transform.All images from foil #4800.(G) Amorphous nano melt inclusions hosted by apatite analyzed in (F); Ca, P, O, F are most likely coming from host apatite.(H) Ilmenite daughter crystals in nano melt inclusions hosted by apatite analyzed in (F); Ca, P, F, S, and Si peaks are most likely coming from the amorphous nano melt inclusions and host apatite; HR-TEM and FFT of ilmenite in Figure S3, M and L. Based on the G and H spectra the composition of the residual melt, i.e., C-O-Si-Cl-Al-HFSE-rich melt, follows the highest to lowest peak counts of glassy nano melt inclusion and daughter ilmenite, discounting the elements that most likely come from the host phases.
Notes: small Cu and Ga peaks in the analyses reflect the composition of Cu TEM grid, and the residual Ga that was used to extract the foils with the focused ion beam (FIB) for the TEM foils, respectively.S9).  ) with plotted re-calculated average compositions of conjugate melts in individual melt inclusions from this study (after 3,4 ).
The data used to represent Fe-rich and Si-rich melts in Fig. 5 a-b includes FEG-EPMA point analyses of cpx from the cpx-mt globules and the high-SiO2 dacite glass enclosed in the melt inclusions, respectively.The cpx composition from cpx-mt globules does not fully reflect the composition of cpx-mt globules, thus Figure S10 also includes recalculated FEG-EPMA point analyses.The re-calculated composition of Fe-rich melt is based on calculated phaseproportions: 82% of cpx FEG-EPMA point analyses (Table S3) and 18 % of stoichiometric mt crystals within cpx-mt globules (Table S6).Similarly, high-SiO2 dacite glass host ~ 5 modal % of cpx crystals, that were included in the Si-rich melt compositions by re-calculating 95% of high-SiO2 dacite glass FEG-EPMA point analyses (Table S2) and 5 % of stoichiometric cpx.The re-calculated compositions of the Fe-rich and Si-rich melt do not vary significantly from the EPMA point analyses plotted on Figure 5a  The average compositions of whole rock (WR) analyses of group 1 andesite (n=7), group 2 andesite (n=9), and dacite (n=2) from El Laco is from 5 .The two groups of andesite are based on variations in the bulk rock Sm-Nd and Rb-Sr isotopic compositions as shown in Figure 2 of 5 .

Table. S1. Average compositions of crystallized melts in melt inclusions and homogenized melt inclusions.
Notes: Re-calculated composition of Fe-rich and Si-rich melt based on 19 and 11 FEG-EPMA analyses, respectively, of clinopyroxene in cpx-mt globules and high-SiO2 dacite glass from eight immiscible melt inclusions and calculation of phase proportions (Table S2-S3, S6).The average composition of homogenized melt inclusions is based on seventy analyses of nineteen homogenized melt inclusions (Table S11).Abbreviations: SDstandard deviation, NAnot applicable.Notes: any analyses that are less than the minimum detection limit (LOD) are listed as <LOD, and the respective errors for those analyses are expressed at 100%; NAnot applicable Notes: any analyses that are less than the minimum detection limit (LOD) are listed as <LOD, and the respective errors for those analyses are expressed at 100%; NAnot applicable Notes: any analyses that are less than the minimum detection limit (LOD) are listed as <LOD, and the respective errors for those analyses are expressed at 100%; NAnot applicable   Notes: any analyses that are less than the minimum detection limit (LOD) are listed as <LOD, and the respective errors for those analyses are expressed at 100%.SDstandard deviation; DLdetection limit Notes: any analyses that are less than the minimum detection limit (LOD) are listed as <LOD, and the respective errors for those analyses are expressed at 100%; SDstandard deviation; DLdetection limit Notes: any analyses that are less than the minimum detection limit (LOD) are listed as <LOD, and the respective errors for those analyses are expressed at 100%.SDstandard deviation; DLdetection limit Notes: any analyses that are less than the minimum detection limit (LOD) are listed as <LOD, and the respective errors for those analyses are expressed at 100%; SDstandard deviation; DLdetection limit Notes: any analyses that are less than the minimum detection limit (LOD) are listed as <LOD, and the respective errors for those analyses are expressed at 100%.SDstandard deviation; DLdetection limit Notes: any analyses that are less than the minimum detection limit (LOD) are listed as <LOD, and the respective errors for those analyses are expressed at 100%; SDstandard deviation; DLdetection limit Notes: any analyses that are less than the minimum detection limit (LOD) are listed as <LOD, and the respective errors for those analyses are expressed at 100%; SDstandard deviation; DLdetection limit Notes: any analyses that are less than the minimum detection limit (LOD) are listed as <LOD, and the respective errors for those analyses are expressed at 100%; SDstandard deviation; DLdetection limit Notes: Major and minor elements were determined using fused bead, followed by acid digestion, and then inductively-coupled plasma-atomic emission spectrometry (ICP-AES; ALS laboratory method ME-ICP06).Loss on ignition (LOI) was determined by heating in an induction furnace (ALS laboratory method OA-GRA05).Total carbon and total sulfur (wt.%) were analyzed by heating in an induction furnace (ALS laboratory method C-IR07 and S-IR08, respectively).A lithium borate flux fusion, followed by acid digestion, was done prior to analysis by inductively coupled plasma mass spectrometry (ICP-MS) for most of the trace elements (ALS laboratory method ME-MS81).As, Bi, Hg, In, Re, Sb, Se, Te, and Tl were separately determined after aqua regia digestion by ICP-MS (ALS laboratory method ME-MS42).Ag, Cd, Co, Cu, Li, Mo, Ni, Sc, Zn were digested in four acids and then analyzed by ICP-AES to (ALS laboratory method ME-4ACD81).

Fig. S1 .
Fig. S1.Transmitted light images of host andesite and plagioclase phenocrysts hosting melt inclusions from this study.(A) Overview of host andesite with plagioclase and minor clinopyroxene phenocrysts.(B-C) Sieve-textured plagioclase phenocrysts with overgrown rims.Resorbed zones contain abundant melt inclusions recording immiscibility between Fe-rich and Si-rich melts.Sample LCO-9.

Fig. S2 .
Fig. S2.Additional TEM images showing textures and mineralogical assemblage in immiscible melt inclusions.(A) HAADF overview image of typical cpx-mt globules in this study.Note two morphologies of clinopyroxene -anhedral crystals in the cpx-mt globules (cpx1) and euhedral clinopyroxene crystals (cpx2) in the high-SiO2 dacite glass.(B) BF image of cpx-mt globules with magnetite (mt) nano-crystals.(C) Close-up HAADF image in A of a cpx-mt globule comprised of cpx1 with euhedral mt crystals precipitated on the surface of the globule.(D) HAADF image of cpx1 and cpx2.(E) HAADF image of an inclusion composed of NaCl crystals in high-SiO2 dacite glass.(F) HAADF image of inclusion of NaCl and Fe-oxide (Fe-ox) crystals, likely hematite, in high-SiO2 dacite glass.(G) BF image of euhedral cpx2 crystals in high-SiO2 dacite glass.

Fig. S4 .
Fig. S4.TEM -EDS analyses of selected phases.(A) Digenite; (B) Magnetite nanocrystal in cpx-mt globules; (C) Clinopyroxene from cpx-mt globules; (D) Euhedral clinopyroxene needles in high-SiO2 dacite glass, (E) High-SiO2 dacite glass.(F) Fluorapatite in cpx-mt globules.(G) Amorphous nano melt inclusions hosted by apatite analyzed in (F); Ca, P, O, F are most likely coming from host apatite.(H) Ilmenite daughter crystals in nano melt inclusions hosted by apatite analyzed in (F); Ca, P, F, S, and Si peaks are most likely coming from the amorphous nano melt inclusions and host apatite; HR-TEM and FFT of ilmenite in Figure S3, M and L. Based on the G and H spectra the composition of the residual melt, i.e., C-O-Si-Cl-Al-HFSE-rich melt, follows the highest to lowest peak counts of glassy nano melt inclusion and daughter ilmenite, discounting the elements that most likely come from the host phases.

Fig. S5 .
Fig. S5.Clinopyroxene ternary diagram of Ca2Si2O6, Mg2Si2O6, and Fe2Si2O6.The FEG-EPMA clinopyroxene point analyses from clinopyroxene-magnetite globules in the immiscible melt inclusions indicate a bimodality in the data; analyses plot primarily in the augite and pigeonite fields.Two analyses are highly depleted in Fe and Mg, and plot outside of clinopyroxene field.Color coding: red squaresclinopyroxene analyses from LCO-1 sample, blue squares -LAC-AND sample, and blue squares -LCO-9 sample.

Fig. S6 .
Fig. S6.Area masks used in calculation of modal percent of different phases.(A) TEM image of globules with Cu-sulfide used in the creation of binary images in ImageJ 1.52a software.(B) Binary image of the whole globules area.(C) Binary image of magnetite crystals area.(D) Binary image of the Cu-sulfide area.Figure associated with the 'Modal percentage and volume calculations of different phases in melt inclusions' section in Methods section.Sample LCO-1_#4796 (TableS9).

Fig. S7 .
Fig. S7.Composition of homogenized melt inclusions obtained by EPMA traverses (example based on sample LCO-1).(A) BSE images showing the EPMA traverses 'C-C' and D-D' crossing two melt inclusions.Only analyses marked as green circles were included into the average composition of homogenized melt inclusions (Table S1 and S11).(B-C) The geochemical composition of analyses of C-C' and D-D' traverses showing the recognized analyses of melt inclusions, albitized zones (AZ), and host plagioclase (plag).
-b and does not change the main results.

Table S2 . Results of FEG-EPMA spot analyses of the high-SiO2 dacite glass in melt inclusions
Notes: any analyses that are less than the minimum detection limit (LOD) are listed as <LOD, and the respective errors for those analyses are expressed at 100%; NAnot applicable

Table S12 .
Whole-rock major and trace elements geochemistry of andesite LCO-9 with plagioclase phenocrysts hosting immiscible melt inclusions used in Figure6cand Supplementary FigureS9C.
1Method name according to ALS laboratory explained below.